1. Field of the Invention
The present invention relates to a system and method for securing Quality of Service (QoS) according to the type of Wireless Local Area Network (WLAN) service in a WLAN communication system based on IEEE 802.11.
2. Description of the Related Art
The existing IEEE 802.11 Media Access Control (MAC) supports a QoS function by a Point Coordination Function (PCF) mechanism which performs polling in a round-robin scheduling method in a Contention Free Period (CFP). However, the IEEE 802.11 MAC has many problems in securing QoS because the PCF occupies a large amount of the bandwidth of the WLAN and cannot exactly anticipate a beacon delay.
At that time when the IEEE 802.11 MAC was designed, various kinds of wireless communication services did not exist and demands on QoS were not much, and thus QoS-related matters did not have a relatively high importance in the IEEE 802.11 MAC. However, various multimedia services such as video services, VoIP telephones, and interactive games have now increased, and it becomes essential to secure QoS in these services. For the foregoing reasons, the IEEE established an IEEE 802.113 standard to secure QoS in the WLAN.
The IEEE 802.11e uses parameter values such as Contention Window (CW, CWmin/CWmax), Backoff Counter (BO), Interframe space-Arbitration Inter Frame Space (AIFS), and transmission opportunity (TXOP) limit to secure QoS in the WLAN. The IEEE 802.11e uses a Hybrid Coordination Function (HCF) mechanism and an Enhanced Distributed Coordination Function (EDCF) mechanism to improve QoS.
For the QoS service, eight User Priorities (UP) and four Access Categories (AC) are used to define a common parameter set for an Enhanced Distributed Channel Access (EDCA). Table 1 below shows a relationship between the user priority and the access category, and Table 2 shows a default EDCA parameter set.
TABLE 1User Priority(UP - Same asAccess CategoryDesignationIEEE 802.1D User Priority)(AC)(Informative)1AC_BKBackground2AC_BKBackground0AC_BEBest Effort3AC_BEBest Effort4AC_VIVideo5AC_VIVideo6AC_VOVoice7AC_VOVoice
TABLE 2TXOP limitDS-CCK/OFDM/OtherACCWminCWmaxAIFSNPBCCCCK-OFDMPHYsAC_BKaCWminaCWmax7000AC_BEaCWminaCWmax3000AC_VI(aCWmin + 1)/aCWmin26.016 ms3.008 ms02 − 1AC_VO(aCWmin + 1)/(aCWmin + 1)/23.008 ms1.504 ms04 − 12 − 1
As shown in Table 1, the IEEE 802.11e standard defines four access categories AC_BK (background), AC_BE (best effort), AC_VI (video), and AC_VO (voice). The four access categories have different parameter values as indicated in Table 2.
In Table 2, when a user sets the access category to the AC_VO, the user has a relatively higher probability of occupying a wireless channel than other users. Thus, users would try to set their access category to the AC_VO having a high priority to increase the probability of occupying a wireless channel. If all the users set to the AC_VO having a high priority, there is no difference in priority among mobile stations located in the same Basic Service Set (BSS), whereby a differentiated QoS service based on the EDCF cannot be provided.